1. Field of Invention
The invention pertains to a system for purification of contaminated liquids. More particularly, this invention pertains to a system for treatment utilizing a plurality of electric-driven membranes and pressure-driven membranes in a plurality of integrated configurations for removal of contaminants and deionization of liquids.
2. Description of the Related Art
In many areas of the world, treatment of saline water and industrial wastewater is necessary to obtain adequate and protect existing supplies of drinking water. In highly developed countries, recycling of waste liquids generated by industry is required by government regulations, and/or is preferred by industry to maximize recovery of useful liquids, to reduce costs of feed liquids, and to minimize waste discharge.
Currently, a number of systems are utilized for desalination and deionization applications, and for treating aqueous waste streams and aqueous/organic mixtures, including membrane-based technologies, distillation and evaporation, and ion exchange. Membrane-based desalting technologies may be categorized as pressure-driven reverse osmosis (RO) and nanofiltration (NF) and electrically-driven electrodialysis (ED). RO, NF, and ED have commonality in that these processes use semi-permeable membranes as key elements in performing the separation, resulting in significant energy savings compared to thermal processes such as distillation or evaporation, and substantial operational cost savings compared to ion exchange resin methods.
The pressure driven processes ultrafiltration (UF), RO, and NF rely on a semi-permeable membrane to separate one component of a solution from another by means of size exclusion, preferential transport, and pressure. UF typically rejects organics over 1,000 molecular weight (MW) while passing ions and small organics along with water, while RO provides separation of both ions and many small organics. NF provides separation in the range between UF and RO. NF membranes have a wide range of performance characteristics but typically reject organic solutes on the order of a nanometer or 10 angstroms in size as well as larger, highly charged multivalent ions such as sulfate and phosphate. NF will typically not efficiently retain or reject smaller species like chloride and organic acids
UF, NF, and RO systems provide varying filtration and separation efficiencies but many times may lack the ability to economically produce a deionized product liquid of sufficient quality or quantity for reuse in industry, discharge, or municipal use; additional treatment may also be required as some components of the liquid may fall outside the operating ranges where separations are the most efficient and economically feasible for these membrane processes
NF and RO processes have been widely utilized for a range of desalination and deionization applications, but product recovery has a major impact on the economics of pressure-driven membranes and limits process applicability. Furthermore, pressure-based membranes have several inherent technical and economical limitations to achieving high feed recoveries, the most severe of which is the osmotic pressure of the feed solution that has to be overcome by the applied hydrostatic (feed) pressure. The osmotic pressure of saline solutions such as brackish water and seawater can be significant. Moreover, since the osmotic pressure is determined by the salt concentration directly at the membrane surface, it can be affected by concentration polarization, which is the build-up of salt near the surface of the membrane due to incomplete mixing of the surface boundary layer fluid with the bulk solution, a phenomenon accentuated by high pressure fluid passing through the membrane material. Although concentration polarization can be minimized by design and operating parameters, it can never be completely excluded and must be overcome by increased applied hydrostatic (feed) pressure, particularly as feed recovery is increased. Overcoming high osmotic pressures and concentration polarization resulting from higher recoveries requires not only substantial energy to produce the necessary higher pressures and flow rates but also additional investment in capital cost for additional membrane area and pumping capacity. It can also result in shorter useful life of the membrane due to compaction effects and enhanced fouling that can occur at higher pressures and recoveries as a result of the concentration of scaling components near the surface of the membrane, particularly for membrane elements near the end of the process line where overall water recoveries are higher. Enhanced fouling increases the required frequency of membrane cleaning, increasing labor and chemical cost, and reducing throughput. For feeds with total dissolved solids (TDS) levels typical of seawater, recoveries approaching and beyond 50% are seldom feasible; for brackish water levels of TDS, recoveries beyond 80% are rarely economical, resulting in substantial waste of pretreated feed that must be returned to the source or alternately disposed.
Furthermore, membrane process equipment size is determined according to feed or concentrate flow requirements and decreases with increased recovery rate and lower feed concentration; conversely, pressure based membranes perform optimally, producing the best product quality and highest permeate flux rates, with low recoveries and low concentration feeds. Energy requirements are also directly related to feed pressures and feed water flow rates necessary to achieve a particular recovery. The design permeate flux rate predicted at a particular recovery likewise affects the number of pressure vessels, manifold connections, and size of membrane skid, as well as the size of the feed water supply systems and pretreatment equipment that are necessary.
Consequently, it is clear that a critical parameter that has the largest effect on investment and operating cost for pressure-driven membrane methods in most applications is the recovery rate ratio of permeate to feed. The feed flow is inversely proportional to the design recovery rate; therefore, the recovery rate directly affects the size and cost of all process equipment and power consumption. Higher recovery rate also contributes to reduced pretreatment capital cost and chemicals used. However, higher recoveries can increase membrane replacement cost as a result of fouling and compaction. Furthermore, pressure based membrane systems inherently perform better at lower feed concentrations and lower recoveries in which the osmotic pressure of the feed and its fouling and scaling potential are minimized.
In an electrodialysis (ED) process, separation, removal, or concentration of ionic species is accomplished by the selective transport of the ions through ion exchange membranes under the influence of an electrical field. Flowing through the series of anion and cation exchange membranes arranged in an alternating pattern between the electrodes having an electrical potential difference, the water diluate (D) feed stream (e.g., seawater for desalination), concentrate (C) stream, and electrode (E) stream are allowed to circulate in the appropriate cell compartments. Under the influence of the electrical potential difference, the negatively charged chlorides, sulfates, and other anions in the diluate (D) stream migrate toward the anode. These ions pass through the positively charged anion exchange membrane, but are rejected by the negatively charged cation exchange membrane and therefore stay in the C stream, which becomes concentrated with the ionic contaminants. The positively charged species such as sodium and other metals in the D stream migrate toward the cathode and pass through the negatively charged cation exchange membrane. These ions also stay in the C stream, being rejected by the anion exchange membrane. The E stream is the electrode stream (e.g., a sodium sulfate solution), which does not become contaminated with any ionic species from the diluate or concentrate streams, although small amounts of hydrogen are generated at the cathode and oxygen at the anode which are subsequently dissipated as the E streams are combined to maintain a neutral pH in the E stream holding tank. The overall result of the ED processing is an ion concentration increase in the concentrate stream with a depletion of ions in the diluted feed stream.
Multi-cell electrodialysis (ED) process stacks are generally built of membrane sheets separated from each other by suitably configured gaskets. For efficient separations, the distance (gap) between the sheets is as small as possible. In most designs, a spacer is introduced between the individual membrane sheets, both to assist in supporting the membrane and to help control the liquid flow distribution. The ED process stacks are typically assembled in the same fashion as a plate-and-frame filter press, the gaskets corresponding to the frames and the membrane sheets corresponding to the plates. The ED process stack configurations include flow channels for distribution of liquids to be treated to each of various layered compartments which are formed by ingenious patterns of mating holes and slots through the gaskets and the membranes prior to assembly of the ED process stack (see U.S. Pat. No. 6,537,436, Schmidt et al.).
In typical ED process stacks, the flow pattern within each compartment (i.e., between any two successive membranes) is determined by the configuration of the gasket and spacer elements used between the membranes. Two distinctively different flow arrangements are typically used. One is known as a tortuous-path design which can incorporate pressure differentials of up to about 125 pounds per square inch between inflow/outflow portions of the ED unit, while the other flow arrangement makes use of a sheet-flow principle which can incorporate pressure differentials up to about 50 pounds per square inch between inflow/outflow portions of the ED unit. ED process stacks include limitations to constant operation at high efficiencies. One design problem for both flow arrangements for multi-membrane and multi-cell stacks is that of assuring uniform fluid flow to the various compartments and effective transport of the separated ionic constituents to the membrane surfaces for removal from the ED process stack. These difficulties are obstacles to economical demineralization.
ED also has inherent limitations, working best at removing low molecular weight ionic components from a feed stream. Non-charged, higher molecular weight, and less mobile ionic species will not typically be significantly removed. This can be a disadvantage when potable water is produced from feed water sources having high suspended solids content or which are contaminated by microorganisms, which would require additional pre-treatment processes for removal prior to ED processing.
Furthermore, the concentration that can be achieved in the ED brine stream (concentrate or “C” stream) is limited by the membrane selectivity loss due to the Donnan exclusion mechanism and water transport from the dilute to the brine caused by osmosis; in particular, at very high concentrations, diffusion of ions from the concentrate stream back into the diluate stream and transport of water across the membranes can offset separation resulting from the applied electric potential, resulting in a poor (i.e., higher ion concentration than desired) product. However, in general, significantly higher brine concentration can be achieved by ED than by RO and the problem of scaling (i.e., precipitation of insoluble di- or multi-valent salts such as calcium sulfate) is less severe in ED than in RO since mono-valent ions are in general transported through the ion exchange membranes faster than multi-valent ions, resulting in a brine less concentrated in the multi-valent ions and so having less scaling potential. In contrast to RO, ED becomes less economical when extremely low salt concentrations in the product are required, as the current density becomes limited and current utilization efficiency decreases as the feed salt concentration becomes lower: with fewer ions in solution to carry current, both ion transport and energy efficiently greatly declines. Consequently, comparatively large membrane areas are required to satisfy capacity requirements for low concentration (and sparingly conductive) feed solutions.
Furthermore, at low feed concentrations, the reduction of ionic concentration polarization becomes an important design issue for ED membranes. Ionic concentration polarization is the reduction of ion concentrations near the membrane surface compared to those in the bulk solution flowing through the membrane compartment. With substantial ionic concentration polarization, electrolytic water splitting occurs due to the deficiency of solute ions adjacent to the membranes that carry the requisite electric current needed for ED membrane operation. The electrolytic water splitting is detrimental to ED process stack efficiency because of the tendency of ionic concentration polarization to occur at the membrane surface due to the hydrodynamic characteristic of channel flow providing thin viscous boundary layers adjacent to confining surfaces (i.e. adjacent membranes). The thin viscous boundary layers impose a resistance to passage of ions much greater than that of a layer of like thickness in a turbulent area of channel flow, and hence increase the likelihood of ionic concentration polarization at the membrane surfaces. Ionic concentration polarization is objectionable due to an inefficient increase in energy consumption without increasing removal of ionic constituents, requiring increased membrane area, along with pH changes in the feed and concentrate streams due to water splitting causing scale deposition in ED stacks.
In general, additional membrane area can be included in an ED process stack to counteract low separation efficiencies. However, the number of cells in an ED stack is limited by practical considerations of assembly and maintenance requirements. Since the failure of a single electrodialysis (ED) membrane can seriously impair stack performance, the necessity to be able to disassemble and reassemble a stack to replace membranes, and the necessity to be able to perform this quickly and easily, effectively limits the number of membranes that can be practically utilized in a stack. As a result, it is often desirable to use several smaller modular-size ED stacks rather than one large ED stack by using several small subassemblies having about 50 to 100 cell pairs (CP), and arranging as many as 10 of these subassemblies in series in a single clamping press. However, such a configuration increases capital costs and makes the process less economically feasible.
An alternative to utilizing modular-size ED stacks or NF or RO alone for separations is to use ED, UF, microfiltration (MF), RO, NF, distillation, evaporation, and other processes in combination with or as a pretreatment in various configurations. However, each process has drawbacks as discussed hereinabove, and prior utilized hybrid systems (e.g., RO coupled with distillation) for increased recovery have been treated as individual unit operations arranged in series sequence, with no interdependence (e.g., RO concentrate only affects operation of the distillation unit, with no reciprocal impact), with each individual process retaining its individual drawback (e.g., low recovery of RO, high operating cost of distillation.
Due to the inadequacies of each of the separate NF, RO and ED treatment systems for deionization, there exists a need for an integrated approach to deionization systems utilizing multiple types of highly efficient liquid treatment subunits including electrodialysis (ED) membrane units operated in integrated configurations with nanofiltration (NF) and/or reverse osmosis (RO) units as determined by an operator, with the feed liquids for each subunit being channeled through at least one mixing unit in order to blend numerous liquid streams into feed liquid streams having constituents optimized for removal of both TDS solids and ionized constituents by the integrated deionization system. The current invention is not a traditional hybrid process, but instead is an integral process, overcoming limitations inherent to both single processes by integrating the two individual unit processes into a single interdependent system. This integrated, interdependent system allows both the pressure-based membranes and ED membranes to operate at the optimum efficiency point of each, with both systems' operation configured to be optimally affected and enhanced by the presence of the other system.